1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
Conventional image forming apparatuses such as electrophotographic printers, copying machines, facsimile machines, and multi-function printers (MFP) employ an electrophotography technology that involves charging, exposing, developing, and transferring. A charging roller charges the surface of a photoconductive drum. An exposing unit illuminates the charged surface for the photoconductive drum in accordance with print data to form an electrostatic latent image on the photoconductive drum. A developing unit develops the electrostatic latent image with toner into a toner image. A transfer roller transfers the toner image onto paper. A fixing unit fuses the toner image on the paper into a permanent image.
After transferring the toner image onto the paper, a cleaning unit removes residual toner from the photoconductive drum.
It is required of a fixing unit that images are fixed at high speed with minimum heat energy. For this purpose, toner having a low glass transition temperature Tg is used for low-temperature fixing.
A toner having a low glass transition temperature Tg tends to cause offset and filming.
Offset is a phenomenon in which when a heat roller applies heat and pressure to the toner on the paper and then the paper is separated from the heat roller, some of the melted toner adheres to the heat roller, and the residual melted toner on the heat roller is then transferred onto the paper. The melted toner adhering to the paper causes unwanted residual images that appear on the paper at intervals of a distance equal to the circumference of the heat roller.
Filming is adhesion of melted toner to the developing blade that occurs due to heat and pressure given to the toner when the toner is rubbed between the developing blade and the developing roller.